I accidentally hit a key on my keyboard (wasn't looking at it) and my priject began playing in reverse. That really blew my mind until I hit the space bar to straighten it out. Still not sure how I did it.
I would guess you hit the J key which is used to scrub backwards. The K key pauses and the L key scrubs forwards. This is only for previewing and finding a spot on the timeline. These keys have no effect on how the video will be rendered.